To prevent wrinkles from arising over a work piece during a press-drawing operation, a press machine is usually equipped with a die cushion below a lower die half.
FIG. 8 shows the structure of a press machine equipped with a die cushion. In the drawing, reference numeral 1 designates a crown, reference numeral 2 designates an upright, reference numeral 3 designates a bed, a reference numeral 4 designates a slide, reference numeral 5 designates an upper die half, reference numeral 6 designates a lower die half, reference numeral 7 designates a bolster, reference numeral 8 designates a moving bolster, reference numeral 9 designates a die cushion pin or plunger, reference numeral 10 designates a pressure equalizing plate, reference numeral 11 designates a pressure equalizing cylinder, reference numeral 12 designates a die cushion pad, reference numeral 13 designates a die cushion leg, reference numeral 14 designates a die cushion air cylinder, reference numeral 15 designates a die cushion rod, and reference numeral 16 designates a damper filled with hydraulic oil.
As shown in FIG. 8, the bolster 7 is mounted on the bed 3 with the moving bolsters 8 interposed therebetween and the die cushion pad 12 is received in the bed 3. The die cushion pad 12 is supported by the die cushion cylinder 14 mounted on the die cushion leg 13. A compressed air supply source is pneumatically connected to the die cushion cylinders 14 via an air pressure regulating unit (not shown).
The lower die half 6 is mounted on the bolster 7 which is formed with pin holes through which a plurality of die cushion pins 9 are inserted. The die cushion pins 9 serve to support a die pad (not shown) received in the lower die half 6.
A plurality of pressure equalizing hydraulic cylinders 11 are mounted on the plate 10 at positions corresponding to the cushion pins 9. As shown in FIG. 9, the pressure equalizing cylinders 11 are arranged in such a manner that the lower end of die cushion pins 9 come in contact with plungers 18. The flange portion 17 of the plungers 18 is received in a cylinder hydraulic chamber and serves as stoppers for preventing the cylinders 11 from moving upward out of the cylinder hydraulic chambers.
The cylinder hydraulic chambers of the pressure equalizing hydraulic cylinders 11 are hydraulically connected to a hydraulic pressure supply source (not shown) via a hydraulic path 19 such as a drilled hole, a pipe, etc. formed in the plate 10. A check valve is provided at the hydraulic path 19 so as to supply hydraulic oil with a high pressure enough to cancel an error in the length of each die cushion pin into the hydraulic path 19 and the respective cylinder chambers.
In performing a press-drawing operation, die cushion pins 9 are selected corresponding to a die assembly and then the press machine is driven. Since the outflow of the pressurized oil supplied into the respective pressure equalizing hydraulic cylinders 11 is blocked, when the slide 4 and the upper die half 5 are lowered and the selected die cushion pins 9 receive a pressing power, the pressing power is transmitted to the die cushion pad 12 via the die cushion pins 9 and the hydraulic cylinders 11 and is absorbed in the die cushion cylinders 14. At this time, uneven distribution of the pressing power due to the unequal length of die cushion pins 9 as well as assembling error of the parts of the press machine is absorbed in the cushioning pressure of each pressure equalizing cylinder 11.
When a die cushion pin 9 is displaced in excess of a maximum displacement in the die cushion pin pressure equalizing system including a plurality of the hydraulic cylinders 11, there appears a problem that the die cushion pin pressure equalizing system cannot perform a pressure equalizing function. This is because the plunger 18 of the hydraulic cylinder 11 in the die cushion pin pressure equalizing system bottoms on the lower surface of the oil chamber for the hydraulic cylinder 11 as shown in the right-hand part of FIG. 9.
To solve the problem of the bottoming, the following measures have conventionally been taken.
(1) Increasing the number of die cushion pins PA1 (2) Reducing air pressure in die cushion cylinders (i.e., die cushion air pressure) to an ultimate extent PA1 (3) Reducing the number of continuous strokes (i.e., the number of strokes per one minute, SPM)
However, (1) is difficult to conduct considering the fact that the number of die cushion pins should correspond to a die assembly, (2) is not suited to conduct because defective pressing is likely to occur and (3) is hardly acceptable to manufactures since a production rate per hour becomes reduced.
In the conventional die cushion pin pressure equalizing system, press-drawing operations have been conducted without taking consideration various working conditions such as diameter of the plunger 18 of each pressure equalizing hydraulic cylinder 11, the number of die cushion pins 9 to be used, a pin touch speed, i.e., speed at which the upper die half 5 comes in contact with the die cushion pins 9, total quantity of hydraulic oil filled in the die cushion pin pressure equalizing system and die cushion air pressure. As a result, excessive peak pressure is generated in the pressure equalizing system and crack or breakage by fatigue occurs at the flange portion of the plunger 18 of unused pressure equalizing hydraulic cylinders 11. These problems are considered to occur because an excessively high intensity of peak oil pressure is generated when the die assembly comes in contact with the die cushion pins 9.
In view of the circumstances as mentioned above, it is preferable not to take the measures (1) to (3) and not to change the peak hydraulic pressure.
The present invention has been made in consideration of the aforementioned background and its object is to provide a method of preventing the plungers of the pressure equalizing hydraulic cylinders provided in a die cushion pin pressure equalizing system from the bottoming without the generation of an excessively high peak pressure and without the reduction of productivity and accuracy in press-drawing operations.